ref(onboarding): Remove unsupported Serverless Cloud

[s1gr1d]

Remove already hidden onboarding guide for Serverless Cloud. This package does not exist anymore: https://www.serverless.com/blog/serverless-cloud-spins-off-as-ampt

ref https://github.com/getsentry/sentry-docs/issues/7718

[github-actions[bot]]

🚨 Warning: This pull request contains Frontend and Backend changes!

It's discouraged to make changes to Sentry's Frontend and Backend in a single pull request. The Frontend and Backend are not atomically deployed. If the changes are interdependent of each other, they must be separated into two pull requests and be made forward or backwards compatible, such that the Backend or Frontend can be safely deployed independently.

Have questions? Please ask in the #discuss-dev-infra channel.

[s1gr1d]

Added frontend/backend PRs: backend: https://github.com/getsentry/sentry/pull/70720 frontend: https://github.com/getsentry/sentry/pull/70719